I did a bit of googling and saw that there are many recipes for making matrix cubes, however, the best ones require 3 of a single color in terms of matrix shards, but all of the guides I have found for them show only two of each shard. Am I missing something here, or are they just really really well hidden?

I've found two yellow shards on my Bounty Hunter, one on Dromund Kaas and the other on Ilum, but no guide I've seen tells me where the final one is. The only other one I've seen in guides is the location of one on Nar Shadaa, but it's in a Republic area I can't access and requires a item from a previous datacron I also cannot access. It's rather frustrating, since I can't find a source that indicates where it might otherwise be. So either I'm reading the directions wrong, the directions are bad, or it is as of yet undiscovered.